In their study researchers followed the progress of 60 Danish men, considered “heavy but healthy” but who wanted to get fit and in “better shape” over three months.

Half of the men were asked to exercise for an hour a day, wearing a heart-rate monitor and calorie counter, while the second group only had to sweat it out for 30 minutes.

His team concluded that just “30 minutes of exercise hard enough to produce a sweat was enough to turn the tide on an unhealthy body mass index”.

“The participants in our study trained every day for three months,” said Mr Rosenkilde, a PhD student from the department of biomedical sciences.

“All training sessions were planned to produce a light sweat, but participants were expected to increase the intensity and give it gas three times a week.

“Participants exercising 30 minutes per day burned more calories than they should relative to the training program we set for them.”

He added: “In fact we can see that exercising for a whole hour instead of a half does not provide any additional loss in either body weight or fat.

“The men who exercised the most lost too little relative to the energy they burned by running, biking or rowing.”

But Mr Rosenkilde admitted he was unsure why the men who exercised longer did not lose more weight.

He said that it may be that they ended up eating more or those who exercised during the 30-minute sessions could have trained for slightly longer because they still had energy left at the end of the session.

He now wants to study the benefits of combining exercise with daily commuting. He added: “If we can get people to exercise along the way to work, for example, we will have won half the battle.”

According to the latest government figures, more than 60 per cent of adults in England and a third of 10 and 11 year-olds are overweight.

The Lancet medical journal has predicted that by 2030 more than 11 million people would be classed as obese, with a BMI above 30. A healthy score is from 18.5 to 25.

Obesity and chronic health conditions such as high blood pressure and diabetes cost Britain £20 billion a year in lost productivity.

NHS research indicates that one in three children leaves primary school either overweight or obese, despite most starting school at a healthy weight.

The Department of Health advises adults that “vigorous exercise” of an hour and a quarter of weekly is needed, such as running or playing football.

It also recommends that two and a half hours of more gentle exercise, such as brisk walking, is also sufficient.